Wireless communication system, base station, mobile station, and wireless communication method

ABSTRACT

A wireless communication system controls a transmission timing in each cell so that a control channel of a first cell and a data channel of a second cell temporally overlap each other. The wireless communication system includes a base station of the first cell. The base station of the first cell includes a first control unit and a first communicating unit. The first control unit notifies a base station of the second cell of information used to specify a resource of the control channel of the first cell. The first communicating unit transmits a control signal to a mobile station of the first cell by using a first resource of the control channel of the first cell.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of International Application PCT/JP2011/051948, filed on Jan. 31, 2011, and designating the U.S., the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are directed to a wireless communication system, a base station, a mobile station, and a wireless communication method.

BACKGROUND

Conventionally, for LTE (Long Term Evolution) or LTE-Advanced as a next generation mobile communication system, a heterogeneous network has been studied in order to improve system capacity and coverage. The heterogeneous network is a network in which a macrocell and a cell of a base station with low transmission power (hereinafter, described as a “picocell”) are arranged so as to coexist with each other. In such a network, when the macrocell and the picocell are operated at the same frequency, interference from the macrocell to the picocell becomes a problem. Namely, in a mobile station connected to a base station of the picocell (hereinafter, described as a “pico base station”), a signal from the pico base station is interfered with by a signal from a base station of the macrocell (hereinafter, described as a “macro base station”).

The inter-cell interference as described above influences the communication quality in each of physical channels (a control channel and a data channel). In particular, in a system in which a transmission timing of a subframe is synchronized between cells, the inter-cell interference may occur between the control channels and between the data channels. As a technology for reducing the inter-cell interference as described above, there is a technology for shifting a transmission timing of a subframe in the pico base station with respect to a transmission timing of a subframe in the macro base station, in an OFDM (Orthogonal Frequency Division Multiplexing) symbol unit. In such a technology, a data channel of the macro base station that overlaps a control channel of the pico base station in the time domain as a result of the shift is overwritten (muting) with a null symbol with transmission power of zero.

-   Non Patent Literature 1: 3GPP TR 36.814 V9.0.0 (2010-03) -   Non Patent Literature 2: 3GPP TS 36.211 V8.9.0 (2009-12) -   Non Patent Literature 3: 3GPP TS 36.213 V8.8.0 (2009-09) -   Non Patent Literature 4: 3GPP R1-103227

However, in the technology described above, while interference from the data channel of the macro base station to the control channel of the pico base station is reduced, there is a problem in that the reception characteristics of the data channel of the macro base station is degraded. Namely, because a null symbol is set in a resource that is muted to reduce the interference among resources of the data channel of the macro base station, the amount of data that the macro base station can transmit per unit time is reduced accordingly. The reduction in the amount of transmission data is a cause of degradation of the reception characteristics of the data channel.

SUMMARY

According to an aspect of the embodiments, a wireless communication system controls a transmission timing in each of cells so that a control channel of a first cell and a data channel of a second cell temporally overlap each other. A base station of the first cell includes a first control unit that notifies a base station of the second cell of information used to specify a resource of the control channel of the first cell, the resource corresponding to a predetermined resource unit; and a first communicating unit that transmits a control signal to a mobile station of the first cell by using a first resource of the control channel of the first cell, the first resource corresponding to at least a part of the predetermined resource unit and serving as a decoding object of the mobile station of the first cell. The base station of the second cell includes a second communicating unit that transmits a null symbol by using a second resource of the data channel of the second cell, the second resource corresponding to the predetermined resource unit. The mobile station of the first cell includes a third communicating unit that receives the control signal transmitted by the base station of the first cell via the first resource and receives the null symbol transmitted by the base station of the second cell via the second resource.

The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an example of a heterogeneous network;

FIG. 2 is a diagram for explaining a mapping method of each physical channel;

FIG. 3 is a diagram for explaining a mapping method of a PDCCH;

FIG. 4 is a diagram for explaining a search space of the PDCCH;

FIG. 5 is a diagram illustrating how a time-frequency resource for a transmission signal of a macro base station and a pico base station is allocated;

FIG. 6 is a diagram for explaining how a frequency resource for a transmission signal of each of the base stations is allocated;

FIG. 7 is a diagram illustrating a configuration of a wireless communication system according to a first embodiment;

FIG. 8 is a diagram illustrating a configuration of a mobile station according to the first embodiment;

FIG. 9 is a diagram illustrating an operation of the wireless communication system according to the first embodiment;

FIG. 10 is a diagram for explaining a PDCCH multiplexing method according to the first embodiment;

FIG. 11 is a diagram for explaining a scheduling algorithm in a pico base station according to the first embodiment;

FIG. 12 is a diagram illustrating a configuration of a wireless communication system according to a fourth embodiment;

FIG. 13 is a diagram illustrating an operation of the wireless communication system according to the fourth embodiment;

FIG. 14 is a diagram illustrating an example of interference reducing subframe information according to the fourth embodiment;

FIG. 15 is a diagram for explaining a scheduling algorithm in a pico base station according to the fourth embodiment;

FIG. 16 is a diagram illustrating a configuration of a wireless communication system according to a fifth embodiment; and

FIG. 17 is a diagram illustrating an operation of the wireless communication system according to the fifth embodiment.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of a wireless communication system disclosed in the present application will be explained in detail below with reference to accompanying drawings. The present invention is not limited to the embodiments below.

First, a technology as a basis of the wireless communication system disclosed in the present application will be explained with reference to FIG. 1 to FIG. 6. FIG. 1 is a diagram illustrating an example of a heterogeneous network. As illustrated in FIG. 1, when a macrocell and a picocell are operated in a mixed manner, a downlink desired signal SG1 from a pico base station to a mobile station connected to the pico base station is greatly influenced by an interference signal SG2 from a macro base station. Therefore, the communication quality is greatly reduced.

Before explanation of an influence of the inter-cell interference on each physical channel, a configuration of each physical channel and a method of mapping to a time-frequency resource will be explained below with reference to FIG. 2. FIG. 2 is a diagram for explaining a mapping method of each physical channel. As illustrated in FIG. 2, a subframe with the length of 1 ms is constituted of 14 OFDM symbols in the time domain, and a control channel is mapped to first n (=1 to 3) OFDM symbols. Examples of the control channel include a PCFICH (Physical Control Format Indicator CHannel), a PHICH (Physical Hybrid ARQ Indicator CHannel), and a PDCCH (Physical Downlink Control CHannel).

The value of n is defined as control information called a CFI (Control Format Indicator). Shared channels PDSCH (Physical Downlink Shared CHannel) used to transmit user data or the like are mapped to the remaining OFDM symbols. In the frequency domain, an RB (Resource Block) serving as a unit for allocating a frequency resource is constituted of 12 subcarriers, and shared channels for each user are frequency-division multiplexed in an RB unit. A cell-specific reference signal (Cell-specific RS (Reference Signal)) used for channel estimation or the like is sparsely mapped in the time and frequency domains. As a minimum unit of the time-frequency resource, an RE (Resource Element) that is a region bounded by one OFDM symbol and one subcarrier is defined. As a mapping unit of the control channel, an REG (Resource Element Group) constituted of four consecutive REs except for the RS in the frequency domain is defined.

Of all the physical channels described above, in particular, a mapping method of the control channel will be explained in detail below. The PCFICH is a physical channel used to transmit a CFI. Four REGs for the PCFICH are mapped so as to be distributed at approximately equal intervals within a system bandwidth with a starting point at the position of a subcarrier dependent on a cell ID in the first OFDM symbol in the subframe.

The PHICH is a physical channel used to transmit ACK/NACK information on an uplink shared channel. The number of PHICH groups is determined depending on a parameter Ng notified by a higher-level layer, and three REGs are used for each of the PHICH groups. The three REGs are mapped so as to be distributed at approximately equal intervals within a system bandwidth with a starting point at the position of the subcarrier dependent on the cell ID among REGs to which the PCFICH is not mapped.

The PDCCH is a physical channel used to transmit notification information and scheduling information on user data. FIG. 3 is a diagram for explaining a method of mapping a PDCCH. A CCE (Control Channel Element) is defined as a resource unit used by each PDCCH. The CCE is constituted of nine REGs (=36 REs). An aggregation level (hereinafter, described as an “AL”) is a parameter corresponding to the number of CCEs used by the PDCCH, that is, a spreading factor. The AL is set to any of {1, 2, 4, 8} by a base station according to the state of a wireless channel or the like. While details will be explained later, each PDCCH is multiplexed by being added with appropriate offset, and is modulated by QPSK (Quadrature Phase Shift Keying). Each PDCCH is interleaved in units of four modulation symbols, and thereafter mapped to an REG to which the PCFICH or the PHICH is not mapped.

Incidentally, the mobile station is not notified of a PDCCH multiplexing position by the base station. Therefore, the mobile station searches for candidates for possible multiplexing positions when decoding the PDCCH, and attempts to decode each receiving signal. To limit the number of times of decoding to the extent that the mobile station can process, a concept of a search space (hereinafter, described as an “SS”) has been introduced. Therefore, the base station multiplexes the PDCCH at an arbitrary location within the limited search space, and the mobile station searches through only the search space for an attempt of decoding.

FIG. 4 is a diagram for explaining a search space of the PDCCH. FIG. 4 illustrates an example of search spaces in a certain subframe when 33 CCEs are available. A common search space (Common Search Space) provided for a PDCCH that transmits the scheduling information of the notification information is always fixed to the first 16 CCEs. The head position of a search space specific to the mobile station (UE (User Equipment) Specific Search Space) provided for the PDCCH that transmits the scheduling information on user data differs for each of the mobile stations, the ALs, and the subframes. The head position is determined by a hash function. The number of available CCEs may change depending on a system bandwidth, an antenna configuration, a CFI, or Ng.

An influence of the inter-cell interference on each of the physical channels will be explained below. FIG. 5 is a diagram illustrating how a time-frequency resource for a transmission signal of the macro base station and the pico base station is allocated. In a system in which a transmission timing of a subframe is synchronized between cells, the inter-cell interference may occur between the shared channels and between the control channels. In LTE (Release-8), an FFR (Fractional Frequency Reuse) technology is applicable in order to reduce the inter-cell interference. For example, regarding the shared channels, specific RBs are allocated to the shared channels of a mobile station at a cell boundary in the pico base station. Meanwhile, in the macro base station, the shared channels are not transmitted by the RBs or transmitted with low transmission power so that the inter-cell interference can be reduced.

However, in the technology described above, it is difficult to apply the FFR to the control channels because the control channels are arranged in a distributed manner in the whole system bandwidth. As a method for reducing the inter-cell interference in the control channels, there is a method as illustrated in FIG. 6. FIG. 6 is a diagram for explaining how a frequency resource for a transmission signal of each base station is allocated. As illustrated in FIG. 6, a transmission timing of the pico base station is first shifted in an OFDM symbol unit with respect to the macro base station. Subsequently, shared channels of the macro base station overlapping control channels of the pico base station are overwritten (muting) with null symbols with transmission power of 0 (zero). Therefore, the interference from the macrocell to the control channels of the picocell can be reduced. However, as described above, the reception characteristics of the shared channels of the macrocell are degraded.

First Embodiment

A configuration of a wireless communication system according to an embodiment disclosed in the present application will be explained below. FIG. 7 is a diagram illustrating a configuration of a wireless communication system according to a first embodiment. As illustrated in FIG. 7, a wireless communication system 1 includes a pico base station 100 and a macro base station 200. The pico base station 100 includes a control unit 100 a and a communicating unit 100 b. The control unit 100 a includes an interference reducing CCE setting unit 101, a scheduler unit 102, a radio resource control unit 103, an inter-cell interference determining unit 104, an uplink control signal demodulating unit 106, and a data signal generating unit 107. The control unit 100 a also includes a control signal generating unit 108, a reference signal generating unit 109, a channel multiplexing unit 110, an IFFT (Inversed Fast Fourier Transform) unit 111, and a transmission timing control unit 112. The communicating unit 100 b includes a reception RF unit 105 and a transmission RF (Radio Frequency) unit 113. All of the components are connected to one another so as to be able to input and output a signal or data unidirectionally or bidirectionally. The control unit 100 a is physically constructed of a digital circuit, a DSP (Digital Signal Processor), a CPU (Central Processing Unit), or the like, and the communicating unit 100 b is physically constructed of an analog circuit including an amplifier and a filter, or the like.

The interference reducing CCE setting unit 101 sets an interference reducing CCE based on a wireless parameter of the picocell, and notifies the scheduler unit 102 of the interference reducing CCE.

The scheduler unit 102 differs from a normal scheduler unit 204 (of the macro base station 200) in that the scheduler unit 102 allocates the CCE to a PDCCH for an interfered pico UE (User Equipment or a mobile station) based on the interference reducing CCE. The scheduler unit 102 determines allocation of a frequency resource to a data signal for each of mobile stations, determines an MCS (Modulation and Coding Scheme), and determines the number of information bits or the like, based on channel quality information (CQI (Channel Quality Indicator)) notified by each of the mobile stations. The scheduler unit 102 also determines the CFI according to the number of the mobile stations, and allocates an available CCE to a PDCCH for each of the mobile stations.

The radio resource control unit 103 notifies a radio resource control unit 205 of the macro base station 200 of a muting request signal and a wireless parameter (the number of antennas, a CFI, a cell ID, or Ng) needed to specify the CCE of the picocell. The notification is performed via a wired interface. The radio resource control unit 103 receives information on the amount of time shift to be described later from the radio resource control unit 205. The radio resource control unit 103 controls handover based on information on the received power (RSRP (Reference Signal Received Power)) of each cell notified by each of the mobile stations.

The inter-cell interference determining unit 104 estimates the state of the inter-cell interference in each of the mobile stations based on the information on the RSRP of each cell notified by each of the mobile stations, and determines whether to request application of muting (an overwriting process with a null symbol). The determination result is transferred, as a muting request signal, to the radio resource control unit 103.

The transmission timing control unit 112 shifts a transmission timing of a downlink signal in an OFDM symbol unit based on the information on the amount of time shift.

Similarly, the macro base station 200 includes a control unit 200 a and a communicating unit 200 b. The control unit 200 a includes a muting processing unit 201, an interference reducing CCE setting unit 202, a muting control unit 203, a scheduler unit 204, a radio resource control unit 205, and an uplink control signal demodulating unit 207. The control unit 200 a also includes a reference signal generating unit 208, a control signal generating unit 209, a data signal generating unit 210, a channel multiplexing unit 211, and an IFFT unit 212. The communicating unit 200 b includes a reception RF unit 206 and a transmission RF unit 213. All of the components are connected to one another so as to be able to input and output a signal or data unidirectionally or bidirectionally. The control unit 200 a is physically constructed of a digital circuit, a DSP, a CPU, or the like, and the communicating unit 200 b is physically constructed of an analog circuit including an amplifier and a filter, or the like.

The muting processing unit 201 performs a muting process based on muting control information transferred by the muting control unit 203.

The interference reducing CCE setting unit 202 sets an interference reducing CCE based on a wireless parameter of the picocell, and notifies the muting control unit 203 of the interference reducing CCE.

The muting control unit 203 determines execution of muting based on the muting request, and sets, as a muting region, a PDSCH RE of a macrocell corresponding to the interference reducing CCE of the picocell. Thereafter, the muting control unit 203 notifies the muting processing unit 201 of muting region information. The muting control unit 203 determines the amount of time shift of the picocell, and notifies the radio resource control unit 205 of the amount of time shift.

The scheduler unit 204 determines allocation of a frequency resource to a data signal for each of the mobile stations, determines an MCS (Modulation and Coding Scheme), and determines the number of information bits or the like, based on the CQI notified by each of the mobile stations. The scheduler unit 204 also determines the CFI according to the number of the mobile stations, and allocates an available CCE to a PDCCH for each of the mobile stations.

The radio resource control unit 205 notifies the radio resource control unit 103 of the pico base station 100 of the information on the amount of time shift via a wired interface. The radio resource control unit 205 receives the muting request signal and the wireless parameter (the number of antennas, a CFI, a cell ID, or Ng) of the picocell from the radio resource control unit 103. The radio resource control unit 205 controls handover based on the information on RSRP of each cell notified by each of the mobile stations.

As other components, the pico base station 100 and the macro base station 200 include a plurality of components that perform processing with common contents. The reception RF units 105 and 206 convert a radio frequency of an uplink received signal into a baseband, and perform quadrature demodulation and A/D (Analog-to-Digital) conversion. The reception RF units 105 and 206 include antennas A1 and A3, respectively, and receive uplink signals. The uplink control signal demodulating units 106 and 207 demodulate uplink control signals and restore the CQI and the RSRP, which are the control information, of each cell. The data signal generating units 107 and 210 generate data signals based on information on the resource allocation, the MCS, or the like. The control signal generating units 108 and 209 generate control signals based on control information containing the resource allocation information or the like. The reference signal generating units 109 and 208 generate reference signals. The channel multiplexing units 110 and 211 multiplex frequencies of the respective physical channels. The IFFT units 111 and 212 perform inverse Fourier transform (IFFT) and adds a CP (Cyclic Prefix). The transmission RF units 113 and 213 perform D/A conversion, quadrature modulation, and conversion from a baseband into a radio frequency, and transmit downlink signals with amplified power. The transmission RF units 113 and 213 include antennas A2 and A4, respectively, and transmit downlink signals.

A configuration of a mobile station 10 will be explained below. FIG. 8 is a diagram illustrating a configuration of the mobile station according to the first embodiment. The mobile station 10 includes a control unit 10 a and a communicating unit 10 b. The control unit 10 a includes an FFT unit 12, a data signal demodulating unit 13, a control signal demodulating unit 14, a channel estimating unit 15, a CQI calculating unit 16, an RSRP measuring unit 17, and an uplink control signal generating unit 18. The communicating unit 10 b includes a reception RF unit 11 and a transmission RF unit 19. All of the components are connected to one another so as to be able to input and output a signal or data unidirectionally or bidirectionally.

The reception RF unit 11 converts a radio frequency of a downlink received signal into a baseband, and performs quadrature demodulation and A/D conversion. The reception RF unit 11 receives a downlink signal via an antenna A5. The FFT (Fast Fourier Transform) unit 12 detects a timing to extract a received signal, removes a CP, and converts the detection result into a received signal in a frequency domain by Fourier transform (FFT), similarly to a typical OFDM system. The data signal demodulating unit 13 demodulates a data signal extracted from the received signal and restores data information based on the resource allocation information. The control signal demodulating unit 14 demodulates a control signal extracted from the received signal and restores the resource allocation information as the control information. The channel estimating unit 15 calculates cross correlation between a reference signal extracted from the received signal and a replica of a known reference signal, to thereby obtain a channel estimation value. The channel estimation is performed not only on a cell to which the mobile station 10 is connected but also on surrounding cells. The CQI calculating unit 16 calculates channel quality information (a CQI as described above) by using the channel estimation value of the cell to which the mobile station 10 is connected. The RSRP measuring unit 17 measures the received power (the RSRP as described above) of the reference signal of each of the cells by using the channel estimation values of the cell to which the mobile station 10 is connected and the surrounding cells. The uplink control signal generating unit 18 generates an uplink control signal based on the control information containing the CQI and the RSRP of each of the cells. The transmission RF unit 19 performs D/A (Digital-to-Analog) conversion and quadrature modulation, and thereafter converts a baseband into a radio frequency and transmits an uplink signal with amplified power. The transmission RF unit 19 transmits an uplink signal via an antenna A6. The control unit 10 a is physically constructed of a digital circuit, a DSP, a CPU, or the like, and the communicating unit 10 b is physically constructed of an analog circuit including an amplifier and a filter, or the like.

An operation will be explained below. In the first embodiment, a network environment is assumed in which one picocell is mixed in a macrocell as illustrated in FIG. 1. FIG. 9 is a diagram illustrating an operation of the wireless communication system 1 according to the first embodiment. In the explanation below, a mobile station connected to the pico base station 100 is described as a pico UE and a mobile station connected to the macro base station 200 is described as a macro UE.

At S1, the pico UE measures the received power of an RS (Reference Signal) of each of the connected cell and the surrounding cells, and notifies the pico base station 100 of the measurement result as the RSRP.

At S2, the pico base station 100 estimates the state of inter-cell interference in each of pico UEs based on the information on the RSRP of each of the cells notified by each of the pico UEs, and determines whether to issue a muting request based on the estimation result. For example, assuming that the RSRP of the picocell as “RSRP_S” and the RSRP of an adjacent cell as “RSRP_I”, a parameter α=RSRP_S/RSRP_I represents the state of the inter-cell interference in the pico UE. Therefore, a mobile station for which α becomes smaller than a predetermined threshold (for example, an SINR (Signal to Interference and Noise Ratio) at which the block error rate of the PDCCH becomes 1% when AL=8 with the greatest resistance against interference is applied) is defined as an interfered pico UE. The number of the interfered pico UEs is used as a criterion, and application of muting is requested when the criterion becomes equal to or greater than a predetermined threshold (for example, 1). The criterion is not limited to the number of the interfered pico UEs but may be a ratio of the number of the interfered pico UEs to the total number of the pico UEs.

At S3, when requesting the application of muting, the pico base station 100 notifies the macro base station 200 of the muting request signal and a wireless parameter (the number of antennas, a CFI, a cell ID, or Ng) needed to recognize the CCE of the picocell.

At S4, the macro base station 200 determines execution of muting based on the muting request signal, and sets a muting region. Specifically, the macro base station 200 calculates the number of CCEs allocatable in the picocell based on the wireless parameter of the picocell. Subsequently, the macro base station 200 defines a specific CCE (for example a common SS) of the picocell as an interference reducing CCE, and sets a corresponding PDSCH RE of the macrocell as the muting region. The macro base station 200 determines the amount of time shift in the picocell. The amount of time shift is determined as, for example, the CFI of the macrocell or the upper-limit value of the CFI, which is 3.

At S5, the macro base station 200 notifies the pico base station 100 of the amount of time shift determined at S4.

At S6, the pico base station 100 changes a transmission timing based on the notified amount of time shift, and sets a CFI update cycle to a value of a longer cycle. This is because, because the muting region of the macrocell is determined based on the CFI of the picocell, if the CFI is frequently changed, recognition of the CFI becomes inconsistent between the macrocell and the picocell. The pico base station 100 also sets a specific CCE (for example, a common SS) as the interference reducing CCE based on a rule shared with the macro base station 200.

FIG. 10 is a diagram for explaining a PDCCH multiplexing method according to the first embodiment. At S7, as illustrated in FIG. 10, the pico base station 100 employs an interfered pico UE as a scheduling object only when a UE-specific SS overlaps an interference reducing CCE, and allocates a part of the CCE in the overlapping region to the PDCCH for the interfered pico UE.

FIG. 11 is a diagram for explaining a scheduling algorithm executed by the pico base station 100 at S7. First, the pico base station 100 selects a candidate UE for scheduling (S11), and determines whether it is possible to ensure a resource for a data signal (S12). As a result of the determination, when it is possible to ensure the resource (YES at S12), the pico base station 100 selects an AL (aggregation level) of the PDCCH (S13), and determines whether the UE selected at S11 is an interfered pico UE (S14).

As a result of the determination at S14, when the UE selected at S11 is not the interfered pico UE (NO at S14), the pico base station 100 determines whether it is possible to ensure a resource for the PDCCH in the SS with the AL selected at S13 (S15). As a result of the determination, when it is possible to ensure the resource (YES at S15), the pico base station 100 ensures the resource for the data signal and the resource for the PDCCH (S16). Thereafter, the pico base station 100 searches for other UEs to be candidates for the scheduling (S17). When there is no other candidate UE (NO at S17), the scheduling process is finished.

As a result of the determination at S14, when the UE selected at S11 is an interfered pico UE (YES at S14), the pico base station 100 determines whether it is possible to ensure a resource for a PDCCH that overlaps an interference reducing CCE in the SS with the AL selected at S13 (S18). As a result of the determination, when it is possible to ensure the resource (YES at S18), the pico base station 100 performs the process at S16 as described above. On the other hand, when it is not possible to ensure the resource at S18 (NO at S18), the process returns to S11 and the subsequent processes are performed again.

When it is determined that it is not possible to ensure the resource at S12 and S15 described above (NO at S12 and NO at S15), the process returns to S11 and the subsequent processes are performed again similarly to the above.

A series of the processes at S11 to S18 described above is repeated until no candidate UE for a scheduling object remains (YES at S17), and is finished when the scheduling process on all of the candidate UEs is completed.

Referring back to FIG. 9, at S8, the pico base station 100 transmits a PDCCH for the interfered pico UE by using the interference reducing CCE.

At S9, the macro base station 200 transmits a PDSCH for the macro UE. At the time of the transmission, the macro base station 200 performs muting on a muting region corresponding to the interference reducing CCE. With the muting, interference from the macrocell to the PDCCH for the interfered pico UE in the picocell is reduced.

The explanation of the operation with reference to the flowchart is complete.

As described above, the wireless communication system 1 of the first embodiment controls a transmission timing in each cell so that the PDCCH of the picocell and the PDSCH of the macrocell temporally overlap each other. The wireless communication system 1 includes the pico base station 100, the macro base station 200, and the pico mobile station 10. The pico base station 100 includes the control unit 100 a and the communicating unit 100 b. The control unit 100 a notifies the macro base station 200 of information used to specify a resource of the PDCCH, where the resource corresponds to a predetermined resource unit. The communicating unit 100 b transmits a control signal to the pico mobile station 10 by using a first resource of the PDCCH of the picocell, where the first resource corresponds to at least a part of the predetermined resource unit and serves as a decoding object of the pico mobile station 10. The macro base station 200 includes the communicating unit 200 b that transmits a null symbol by using a second resource of the PDSCH of the macrocell, where the second resource corresponds to the predetermined resource unit. The pico mobile station 10 includes the communicating unit 10 b that receives the control signal transmitted by the pico base station 100 via the first resource, and that receives a null symbol transmitted by the macro base station 200 via the second resource. Incidentally, the predetermined resource unit is one or more CCEs, and is, for example, an interference reducing CCE. The first resource is a search space specific to the pico mobile station 10, and is, for example, a resource element to which a CCE in a region where the interference reducing CCE and a user-specific search space overlaps each other is mapped in a time-frequency domain. The second resource is a resource element to which the interference reducing CCE is mapped in a time-frequency domain. Therefore, the wireless communication system 1 can reduce interference with the PDCCH while maintaining the reception characteristics of the PDSCH.

Second Embodiment

In a second embodiment, an example will be explained in which a TPC (Transmission Power Control) technology is applied to the wireless communication system of the first embodiment. Specifically, the scheduler unit 102 of the pico base station 100 of the first embodiment employs an interfered pico UE as a scheduling object only when the UE-specific SS overlaps the interference reducing CCE. However, because the head position of the UE-specific SS differs for each of the mobile stations, the ALs, and the subframes, the SS for an optimal AL for a wireless channel state does not always overlap the interference reducing CCE.

Therefore, if it is possible to select an AL other than the optimal AL for the wireless channel state, it becomes possible to improve the probability that the UE-specific SS overlaps the interference reducing CCE. Consequently, it becomes possible to increase opportunities to perform scheduling on the interfered pico UE, so that the throughput may be improved. However, it is concerned that a block error of the PDDCH frequently occurs if an AL other than the optimal AL for the wireless channel state is simply applied.

Therefore, a wireless communication system according to the second embodiment concurrently performs transmission power control (TPC) to avoid a concerning situation as described above. For example, assuming that the optimal AL for the wireless channel state is AL_(opt) and the AL actually applied is AL_(sel), the scheduler unit 102 multiplies the transmission power per RE by (AL_(opt)/AL_(sel)). Therefore, the transmission power per PDCCH resource becomes equal to the transmission power obtained when AL=AL_(opt) is applied. As a result, the block error of the PDCCH can be suppressed.

As described above, in the wireless communication system 1 of the second embodiment, the control unit 100 a of the pico base station 100 provisionally selects an aggregation level to be applied to the PDCCH for the mobile station 10 connected to the picocell based on the wireless channel state of the mobile station 10. When a UE-specific SS corresponding to the provisional aggregation level of the mobile station 10 does not overlap the interference reducing CCE, the communicating unit 100 b of the pico base station 100 selects an aggregation level, for which the UE-specific SS overlaps the interference reducing CCE and which is equal to or greater than the provisional value, and performs transmission. Alternatively, the communicating unit 100 b selects an aggregation level, for which the UE-specific SS overlaps the interference reducing CCE and which is smaller than the provisional value, and performs transmission with transmission power adjusted to be equal to or greater than a predetermined value. Consequently, it is possible to suppress the block error of the PDCCH and improves the probability that the UE-specific SS overlaps the interference reducing CCE. As a result, it is possible to increase opportunities to perform scheduling on the interfered pico UE, so that the system throughput can be improved.

Third Embodiment

In a third embodiment, an example will be explained in which a bulk muting technology for a plurality of pico base stations is applied to the wireless communication system of the first embodiment. In the third embodiment, a network environment is assumed in which a plurality of picocells are mixed in a macrocell, which is different from the first embodiment.

In the third embodiment, it is concerned how the muting control unit 203 of the macro base station 200 determines whether muting is actually applied based on the muting request signal notified by a plurality of the pico base stations. A criterion for the determination may be based on whether the number of pico base stations that have requested application of muting exceeds a predetermined threshold.

As a first example, there is a case in which an emphasis is placed on reduction of interference from the macro base station 200 to the control channel of the pico base station 100 as a design principle of the mobile communication system. In this case, it is sufficient that the muting control unit 203 actually applies muting when one or more pico base stations have requested application of muting.

As a second example, there is a trade-off relation of the transmission efficiencies of the pico base station 100 and the macro base station 200. Specifically, while the reception characteristics of the pico base station 100 improves with an increase in the amount of muting, the reception characteristics of the macro base station 200 is sacrificed. In view of the above circumstances, there is a case in which an emphasis is placed on balancing between the characteristics. In this case, it is sufficient that the muting control unit 203 actually applies muting when over half of the pico base stations have requested application of muting.

A point that needs to be considered in the third embodiment is how to set interference reducing CCEs in a plurality of the pico base stations and how to set a muting region in the macro base station 200. For example, the muting control unit 203 defines a specific CCE (for example, a common SS) of each of the picocells as an interference reducing CCE similarly to the first embodiment. A correspondence between the CCE and the position of an RE to which the CCE is mapped depends on the number of transmission antennas, a CFI, a cell ID, and Ng of each of the picocells. Therefore, the correspondence is not always the same between the picocells. Therefore, it is sufficient that the muting control unit 203 sets a muting region so as to include all of the REs to which the interference reducing CCEs of all of the picocells are mapped.

As described above, in the wireless communication system 1 according to the third embodiment, the macro base station 200 includes the control unit 200 a and the communicating unit 200 b. The control unit 200 a calculates, for each of the picocells, an RE to which the interference reducing CCE of the picocell is mapped based on the wireless parameter of each of the picocells. The communicating unit 200 b transmits a null symbol in each of the REs calculated by the control unit 200 a by using the same subframe. Therefore, in the network environment in which a plurality of the picocells are mixed in the macrocell, a bulk muting technology for a plurality of the pico base stations can be applied to the wireless communication system of the first embodiment. As a result, it is possible to reduce interference from a single macrocell to the PDCCHs of the picocells.

In the wireless communication system 1 according to the third embodiment, the control unit 100 a of the pico base station 100 notifies the macro base station 200 of a wireless parameter of the picocell as information needed to specify a resource to which the interference reducing CCE is mapped. The control unit 200 a of the macro base station 200 specifies the resource to which the interference reducing CCE of the picocell is mapped based on the information on the wireless parameter notified by the pico base station 100. Therefore, the macro base station 200 can accurately recognize the position of an RE to which the PDCCH of the picocell is highly likely to be mapped.

Fourth Embodiment

In a fourth embodiment, an example will be explained in which a time-division muting technology for a plurality of pico base stations is applied to the wireless communication system of the first embodiment. In the fourth embodiment, a network environment is assumed in which a plurality of picocells are mixed in a macrocell, which is different from the first embodiment.

In the third embodiment, the muting control unit 203 sets a muting region of the macrocell so as to include all of the REs to which the interference reducing CCEs of all of the picocells are mapped. Therefore, it becomes possible to collectively control interference with the interfered pico UEs in a plurality of the picocells. However, a needed muting region tends to be greater. Therefore, in the fourth embodiment, the wireless communication system controls interference with the interfered pico UEs in a plurality of picocells while maintaining a small needed muting region. To control the interference as described above, the muting control unit of the macro base station performs muting specifically on an individual picocell for each subframe.

A configuration of the wireless communication system according to the fourth embodiment will be explained below. FIG. 12 is a diagram illustrating a configuration of the wireless communication system according to the fourth embodiment. As illustrated in FIG. 12, a wireless communication system 2 includes a pico base station 300 and a macro base station 400. The pico base station 300 includes a control unit 300 a and a communicating unit 300 b. The control unit 300 a includes an interference reducing CCE setting unit 301, a scheduler unit 302, a radio resource control unit 303, an inter-cell interference determining unit 304, an uplink control signal demodulating unit 306, and a data signal generating unit 307. The control unit 300 a also includes a control signal generating unit 308, a reference signal generating unit 309, a channel multiplexing unit 310, an IFFT unit 311, and a transmission timing control unit 312. The communicating unit 300 b includes a reception RF unit 305 and a transmission RF unit 313. All of the components are connected to one another so as to be able to input and output a signal or data unidirectionally or bidirectionally.

Similarly, the macro base station 400 includes a control unit 400 a and a communicating unit 400 b. The control unit 400 a includes a muting processing unit 401, an interference reducing CCE setting unit 402, a muting control unit 403, a scheduler unit 404, a radio resource control unit 405, and an uplink control signal demodulating unit 407. The control unit 400 a also includes a reference signal generating unit 408, a control signal generating unit 409, a data signal generating unit 410, a channel multiplexing unit 411, and an IFFT unit 412. The communicating unit 400 b includes a reception RF unit 406 and a transmission RF unit 413. All of the components are connected to one another so as to be able to input and output a signal or data unidirectionally or bidirectionally.

The wireless communication system 2 has the same configuration as that of the wireless communication system 1 of the first embodiment. Therefore, the same components are denoted by reference symbols with the same trailing numerals, and detailed explanation thereof will be omitted.

Specifically, the pico base station 300 and the macro base station 400 of the fourth embodiment are components corresponding to the pico base station 100 and the macro base station 200 of the first embodiment, respectively. The control unit 300 a and the communicating unit 300 b of the pico base station 300 correspond to the control unit 100 a and the communicating unit 100 b of the pico base station 100, respectively. Similarly, the control unit 400 a and the communicating unit 400 b of the macro base station 400 correspond to the control unit 200 a and the communicating unit 200 b of the macro base station 200, respectively.

The interference reducing CCE setting unit 301, the scheduler unit 302, and the radio resource control unit 303 of the pico base station 300 correspond to the interference reducing CCE setting unit 101, the scheduler unit 102, and the radio resource control unit 103 of the pico base station 100, respectively. The inter-cell interference determining unit 304, the reception RF unit 305, the uplink control signal demodulating unit 306, and the data signal generating unit 307 correspond to the inter-cell interference determining unit 104, the reception RF unit 105, the uplink control signal demodulating unit 106, and the data signal generating unit 107, respectively. The control signal generating unit 308, the reference signal generating unit 309, and the channel multiplexing unit 310 correspond to the control signal generating unit 108, the reference signal generating unit 109, and the channel multiplexing unit 110, respectively. The IFFT unit 311, the transmission timing control unit 312, and the transmission RF unit 313 correspond to the IFFT unit 111, the transmission timing control unit 112, and the transmission RF unit 113, respectively.

Similarly, the muting processing unit 401 and the interference reducing CCE setting unit 402 of the macro base station 400 correspond to the muting processing unit 201 and the interference reducing CCE setting unit 202 of the macro base station 200, respectively. The muting control unit 403, the scheduler unit 404, and the radio resource control unit 405 correspond to the muting control unit 203, the scheduler unit 204, and the radio resource control unit 205, respectively. The reception RF unit 406, the uplink control signal demodulating unit 407, and the reference signal generating unit 408 correspond to the reception RF unit 206, the uplink control signal demodulating unit 207, and the reference signal generating unit 208, respectively. The control signal generating unit 409, the data signal generating unit 410, and the channel multiplexing unit 411 correspond to the control signal generating unit 209, the data signal generating unit 210, and the channel multiplexing unit 211, respectively. The IFFT unit 412 and the transmission RF unit 413 correspond to the IFFT unit 212 and the transmission RF unit 213, respectively.

The configuration of the mobile station is the same as that of the first embodiment, and therefore, explanation thereof will be omitted.

A main difference between the fourth embodiment and the first embodiment will be explained below. The inter-cell interference determining unit 304 of the pico base station 300 estimates a state of inter-cell interference in each of the mobile stations based on information on the RSRP of each cell notified by each of the mobile stations. The inter-cell interference determining unit 304 determines whether to request application of muting based on the estimation result, and generates information on the number of interfered UEs. The inter-cell interference determining unit 304 transfers the information on the number of the interfered UEs to the radio resource control unit 303. The radio resource control unit 303 of the pico base station 300 notifies the radio resource control unit 405 of the macro base station 400 of information on the number of the interfered UEs and a wireless parameter (the number of antennas, a CFI, a cell ID, or Ng) needed to recognize the CCE of the picocell. The radio resource control unit 303 receives information on the amount of time shift and interference reducing subframe information for each of the picocells from the macro base station 400. The scheduler unit 302 of the pico base station 300 allocates a CCE to a PDCCH for the interfered pico UE based on the interference reducing subframe information for each of the picocells and the interference reducing CCE.

Meanwhile, the muting control unit 403 of the macro base station 400 generates interference reducing subframe information for each of the picocells based on the information on the number of the interfered UEs for each of the picocells. The muting control unit 403 sets a PDSCH RE of a macrocell corresponding to the interference reducing CCE of each of the picocells as the muting region for each of the picocells. The muting control unit 403 notifies the muting processing unit 401 of the muting region information for the picocell, for which an interference reducing subframe is set, for each subframe.

The muting control unit 403 determines the amount of time shift common to all of the picocells, and notifies the radio resource control unit 405 of the amount of time shift together with the interference reducing subframe information for each of the picocells.

The radio resource control unit 405 of the macro base station 400 notifies the radio resource control unit 303 of each of the pico base stations 300 of the information on the amount of time shift and the interference reducing subframe information for each of the picocells, via a wired interface. The radio resource control unit 405 receives the information on the number of interfered UEs and the wireless parameter (the number of antennas, a CFI, a cell ID, or Ng) of each of the picocells from the pico base stations 300. The radio resource control unit 405 also transmits and receives various types of data and signals to and from a pico base station 500.

An operation will be explained below. In the fourth embodiment, a network environment is assumed in which two picocells are mixed in a macrocell. FIG. 13 is a diagram illustrating an operation of the wireless communication system 2 according to the fourth embodiment. In the explanation below, a mobile station connected to the pico base station 300 is described as a pico UE 10, a mobile station connected to the pico base station 500 is described as a pico UE 20, and a mobile station connected to the macro base station 400 is described as a macro UE. Furthermore, a picocell formed by the pico base station 300 is described as a picocell C1, and a picocell formed by the pico base station 500 is described as a picocell C2.

At S21, the pico UE 10 measures the received power of the RS of each of the connected cell and the surrounding cells, and notifies the pico base station 300 of the measurement result as the RSRP. The pico UE 20 performs the same process as S21, and notifies the pico base station 500 of the RSRP of each of the cells (S22).

At S23, the pico base station 300 estimates the state of the inter-cell interference of each of the pico UEs based on the information on the RSRP of each of the cells notified by each of the pico UEs, and determines whether to request muting based on the estimation result. The pico base station 500 performs the same estimation process and the determination process (S24). The detailed processing contents are the same as those of the process at S2 in FIG. 9 of the first embodiment, and therefore, detailed explanation thereof will be omitted.

At S25, when requesting application of the muting, the pico base station 300 notifies the macro base station 400 of information on the number of interfered UEs and a wireless parameter (the number of antennas, a CFI, a cell ID, or Ng) needed to recognize the picocell C1. Incidentally, as the information on the number of the interfered UEs, the number of the interfered pico UEs is used that serves as a criterion for determining whether to request muting. The information on the number of the interfered UEs is not limited to the above number, and may be a ratio of the number of the interfered pico UEs to the total number of the pico UEs. At S26, the pico base station 500 performs the same process as S25 on the macro base station.

At S27, the macro base station 400 determines a method for applying the muting based on the information on the number of the interfered UEs and the picocell information obtained from the pico base stations 300 and 500. Specifically, the macro base station 400 defines an interference reducing CCE for each of target picocells, and sets a corresponding PDSCH RE of the macrocell as a muting region. The macro base station 400 also determines the interference reducing CCE for the target picocell and a subframe (interference reducing subframe) in which the muting region is set, based on the information on the number of the interfered UEs. For example, when values indicated by the information on the number of the interfered UEs of the picocells C1 and C2 are p_mute1 and p_mute2, respectively, the macro base station 400 determines the ratio of the number of the interference reducing subframes of the picocells C1 and C2 as (p_mute1:p_mute2). The macro base station 400 generates information (interference reducing subframe information) indicating the position of the interference reducing subframe for each of the picocells C1 and C2 in the subframe based on the above ratio.

FIG. 14 is a diagram illustrating an example of the interference reducing subframe information according to the fourth embodiment. FIG. 14 illustrates the interference reducing subframe information defined by a 10-subframe cycle when (p_mute1:p_mute2)=(3:7). In FIG. 14, in the subframe number for which “1” is assigned, an interference reducing subframe for a corresponding picocell is set. The macro base station 400 determines the amount of time shift common to the picocells C1 and C2. The amount of time shift is determined as, for example, the CFI of the macrocell or the upper-limit value of the CFI, which is 3.

Referring back to FIG. 13, at S28, the macro base station 400 notifies the pico base station 500 of the interference reducing subframe information and the amount of time shift determined at S27. The pieces of the information are also sent from the macro base station 400 to the pico base station 300 (S29).

At S30, the pico base station 300 changes a transmission timing based on the amount of time shift notified by the macro base station 400 and sets the CFI update cycle to a value of a longer cycle. The pico base station 300 sets a specific CCE (for example, a common SS) as the interference reducing CCE of the picocell C1 based on a rule shared with the macro base station 400.

At S31, only when the interference reducing subframe for a picocell is set with respect to the interfered pico UE and the UE-specific SS overlaps the interference reducing CCE of the picocell C1, the pico base station 300 applies the interfered pico UE as a scheduling object. The pico base station 300 allocates a part of the CCE in the overlapping region to the PDCCH for the interfered pico UE.

A scheduling algorithm according to the fourth embodiment will be explained below. FIG. 15 is a diagram for explaining the scheduling algorithm of the pico base stations 300 and 500 according to the fourth embodiment. FIG. 15 is the same as FIG. 11 except for a determination process that is newly added at Step S49. Therefore, detailed explanation of FIG. 15 will be omitted. Steps S41 to S48 in FIG. 15 correspond to Steps S11 to S18 in FIG. 11, respectively.

In the first embodiment, it has been explained that the determination on whether the selected candidate UE is an interfered pico UE (S14 in FIG. 11) is performed, and thereafter determination on whether it is possible to ensure a resource for the PDCCH is performed regardless of a result of the former determination (S15 or S18). By contrast, in the fourth embodiment, when the UE selected at S41 is an interfered pico UE (YES at S44), each of the pico base stations 300 and 500 determines whether a corresponding subframe is an interference reducing subframe for a corresponding picocell (S49). As a result of the determination, when the subframe is the interference reducing subframe (YES at S49), the process proceeds to S48. On the other hand, when the subframe is not the interference reducing subframe (NO at S49), the process returns to S41 and the processes from S41 are performed again. The processes to be performed when the selected candidate UE is not the interfered pico UE at S44 (NO at S44) are the same as those of the first embodiment.

Referring back to FIG. 13, the pico base station 500 performs the same processes as S30 and S31 described above, based on the interference reducing subframe information and the amount of time shift notified at S28 (S32 and S33).

At S34, the pico base station 300 transmits a PDCCH for the interfered pico UE in the interference reducing subframe for the picocell C1 by using the interference reducing CCE for the picocell C1. At the same time, when the macro base station 400 transmits a PDSCH, muting is performed on the muting region corresponding to the interference reducing CCE for the picocell C1 (S35). Therefore, interference from the macrocell to the PDCCH for the interfered pico UE in the picocell C1 can be reduced.

At S36, the pico base station 500 transmits a PDCCH for the interfered pico UE in the interference reducing subframe for the picocell C2 by using the interference reducing CCE for the picocell C2. At the same time, when the macro base station 400 transmits a PDSCH, muting is performed on a muting region corresponding to the interference reducing CCE corresponding to the picocell C2 (S37). Therefore, interference from the macrocell to the PDCCH for the interfered pico UE in the picocell C2 can be reduced.

In the fourth embodiment, the picocells C1 and C2 have been explained as representative picocells. Meanwhile, the positions of REs to which the PDCCHs of picocells other than the representative picocells are mapped partially overlap the muting region. Therefore, it is also possible to partially reduce interference with the PDCCH.

As described above, the wireless communication system 2 of the fourth embodiment includes a plurality of the pico base stations 300 and 500. The pico base station 300 includes the control unit 300 a and the communicating unit 300 b. The control unit 300 a notifies the macro base station 400 of the information needed to specify a CCE to which the interference reducing CCE is mapped and the information on the number of interfered UEs. A case is assumed in which UE-specific search spaces of mobile stations connected to the picocells C1 and C2 overlap the interference reducing CCE in the interference reducing subframes for the picocells C1 and C2 indicated by the interference reducing subframe information notified by the macro base station 400. In this case, the communicating unit 300 b performs transmission by a PDCCH for the mobile station by using the interference reducing CCE in the overlapping region. The macro base station 400 includes the control unit 400 a and the communicating unit 400 b. The control unit 400 a sets the interference reducing subframe for each of the picocells C1 and C2 based on the information on the number of the interfered UEs notified by a plurality of the pico base stations 300 and 500, and notifies each of the pico base stations 300 and 500 of the interference reducing subframe information. The communicating unit 400 b transmits a null symbol in a resource to which the interference reducing CCE of the picocell is mapped in the interference reducing subframe for each of the picocells C1 and C2. Therefore, in the network environment in which a plurality of the picocells C1 and C2 is mixed in a macrocell, it is possible to apply the time-division muting technology to the wireless communication system of the first embodiment. Therefore, interference from a single macrocell to the PDCCHs of the picocells C1 and C2 can be reduced. In addition, the reception characteristics of the PDSCH of the macrocell can be maintained regardless of the number of the picocells.

In the wireless communication system 2, the control unit 300 a of the pico base station 300 notifies the macro base station 400 of information based on the number of the UEs in the wireless channel state of a predetermined quality or lower, as the information on the number of the interfered UEs. The wireless channel state of the predetermined quality or lower indicates, for example, a state in which a measured reception level is equal to or lower than a predetermined value. Therefore, the control unit 300 a can issue more appropriate information indicating how many mobile stations are greatly interfered.

In the wireless communication system 2, the control unit 400 a of the macro base station 400 sets a ratio of the picocells C1 and C2 with respect to the number of the interference reducing subframes based on the ratio of the picocells C1 and C2 with respect to values indicated by the information on the number of the interfered UEs. Therefore, it is possible to prevent inequality of opportunity for transmission of the PDCCHs of the picocells C1 and C2 between the cells.

Furthermore, in the wireless communication system 2, the control unit 400 a of the macro base station 400 sets the interference reducing subframes of the picocells C1 and C2 so that the subframes do not overlap each other. Therefore, the wireless communication system 2 can assuredly apply an optimal interference control process for each of the picocells in each of the subframes.

Moreover, in the wireless communication system 2, the communicating unit 400 b of the macro base station 400 transmits a null symbol to the mobile station 10 of the pico base station 300 in response to a request from a pico base station that has issued a request for transmission of the null symbol from among a plurality of the pico base stations 300 and 500. Therefore, muting is performed only on a pico base station for which interference needs to be controlled. Therefore, compared with a case in which the muting is performed on all of the pico base stations, it is possible to reduce processing load on the macro base station 400 due to control of the interference.

Fifth Embodiment

In a fifth embodiment, an example will be explained in which a technology for adaptively controls the interference reducing CCE is applied to the wireless communication system of the first embodiment. Specifically, in the first embodiment, the wireless communication system 1 sets the interference reducing CCE in, for example, the common SS according to the rule common to the macro base station and the pico base station. However, if the interference reducing CCE is fixedly set, when, for example, the number of the interfered pico UEs in the picocell increases, it becomes difficult for the wireless communication system 1 to ensure resources for the PDCCHs of the interfered pico UEs and the throughput of the interfered pico UEs may be reduced. On the other hand, when the number of the interfered pico UEs is small, while the wireless communication system 1 can easily ensure a resource for the PDCCHs of the interfered pico UEs, the number of the PDSCH REs muted in the macrocell becomes excessive. Therefore, a wireless communication system according to a fifth embodiment controls the interference reducing CCE according to the communication status of the picocell.

A configuration of the wireless communication system according to the fifth embodiment will be explained below. FIG. 16 is a diagram illustrating the configuration of the wireless communication system according to the fifth embodiment. As illustrated in FIG. 16, a wireless communication system 3 includes a pico base station 600 and a macro base station 700. The pico base station 600 includes a control unit 600 a and a communicating unit 600 b. The control unit 600 a includes a scheduler unit 602, a radio resource control unit 603, an inter-cell interference determining unit 604, an uplink control signal demodulating unit 606, and a data signal generating unit 607. The control unit 600 a also includes a control signal generating unit 608, a reference signal generating unit 609, a channel multiplexing unit 610, an IFFT unit 611, and a transmission timing control unit 612. The communicating unit 600 b includes a reception RF unit 605 and a transmission RF unit 613. All of the components are connected to one another so as to be able to input and output a signal or data unidirectionally or bidirectionally.

Similarly, the macro base station 700 includes a control unit 700 a and a communicating unit 700 b. The control unit 700 a includes a muting processing unit 701, an interference reducing CCE setting unit 702, a muting control unit 703, a scheduler unit 704, a radio resource control unit 705, and an uplink control signal demodulating unit 707. The control unit 700 a also includes a reference signal generating unit 708, a control signal generating unit 709, a data signal generating unit 710, a channel multiplexing unit 711, and an IFFT unit 712. The communicating unit 700 b includes a reception RF unit 706 and a transmission RF unit 713. All of the components are connected to one another so as to be able to input and output a signal or data unidirectionally or bidirectionally.

The wireless communication system 3 has the same configuration as that of the wireless communication system 1 of the first embodiment. Therefore, the same components are denoted by reference symbols with the same trailing numerals, and explanation thereof will be omitted.

Specifically, the pico base station 600 and the macro base station 700 of the fifth embodiment are components corresponding to the pico base station 100 and the macro base station 200 of the first embodiment, respectively. The control unit 600 a and the communicating unit 600 b of the pico base station 600 correspond to the control unit 100 a and the communicating unit 100 b of the pico base station 100, respectively. Similarly, the control unit 700 a and the communicating unit 700 b of the macro base station 700 correspond to the control unit 200 a and the communicating unit 200 b of the macro base station 200, respectively.

The scheduler unit 602 and the radio resource control unit 603 of the pico base station 600 correspond to the scheduler unit 102 and the radio resource control unit 103 of the pico base station 100, respectively. The inter-cell interference determining unit 604, the reception RF unit 605, the uplink control signal demodulating unit 606, and the data signal generating unit 607 correspond to the inter-cell interference determining unit 104, the reception RF unit 105, the uplink control signal demodulating unit 106, and the data signal generating unit 107, respectively. The control signal generating unit 608, the reference signal generating unit 609, and the channel multiplexing unit 610 correspond to the control signal generating unit 108, the reference signal generating unit 109, and the channel multiplexing unit 110, respectively. Furthermore, the IFFT unit 611, the transmission timing control unit 612, and the transmission RF unit 613 correspond to the IFFT unit 111, the transmission timing control unit 112, and the transmission RF unit 113, respectively.

Similarly, the muting processing unit 701 and the interference reducing CCE setting unit 702 of the macro base station 700 correspond to the muting processing unit 201 and the interference reducing CCE setting unit 202 of the macro base station 200, respectively. The muting control unit 703, the scheduler unit 704, and the radio resource control unit 705 correspond to the muting control unit 203, the scheduler unit 204, and the radio resource control unit 205, respectively. The reception RF unit 706, the uplink control signal demodulating unit 707, and the reference signal generating unit 708 correspond to the reception RF unit 206, the uplink control signal demodulating unit 207, and the reference signal generating unit 208, respectively. The control signal generating unit 709, the data signal generating unit 710, and the channel multiplexing unit 711 correspond to the control signal generating unit 209, the data signal generating unit 210, and the channel multiplexing unit 211, respectively. The IFFT unit 712 and the transmission RF unit 713 correspond to the IFFT unit 212 and the transmission RF unit 213, respectively.

The configuration of the mobile station is the same as that of the first embodiment, and therefore, explanation thereof will be omitted.

A main difference between the fifth embodiment and the first embodiment will be explained below. The interference reducing CCE setting unit 702 of the macro base station 700 adjusts the interference reducing CCE based on information on the number of interfered UEs and picocell information, and notifies the muting control unit 703 and the radio resource control unit 705 of the adjustment result as interference reducing CCE information. The radio resource control unit 705 of the macro base station 700 notifies the radio resource control unit 603 of the pico base station 600 of the amount of time shift and the interference reducing CCE information. The radio resource control unit 603 of the pico base station 600 notifies the scheduler unit 602 of the interference reducing CCE information.

An operation will be explained below. FIG. 17 is a diagram illustrating an operation of the wireless communication system 3 according to the fifth embodiment. FIG. 17 is the same as FIG. 9 except for Steps S53 to S55. Therefore, detailed explanation of FIG. 17 will be omitted and a difference from the first embodiment will be explained below. Steps S51 to S59 in FIG. 17 correspond to Step S1 to S9 illustrated in FIG. 9, respectively.

At S53, the pico base station 600 notifies the macro base station 700 of the picocell information (the number of antennas, a CFI, a cell ID, and Ng) and information on the number of interfered UEs (mobile stations). The macro base station 700 adjusts the number of the interference reducing CCEs based on the information on the number of the interfered UEs notified by the pico base station 600 (S54). Specifically, when the number of the interfered UEs indicated by the information on the number of the interfered UEs or the ratio of the number of the interfered UEs is equal to or greater than a predetermined value, the macro base station 700 increases the number of the interference reducing CCEs. On the other hand, when the number of the interfered UEs indicated by the information on the number of the interfered UEs or the ratio of the number of the interfered UEs is smaller than the predetermined value, the macro base station 700 decreases the number of the interference reducing CCEs. At S55, the macro base station 700 notifies the pico base station 600 of the information on the number of the interference reducing CCEs adjusted at S54, as interference reducing CCE information, together with the amount of time shift.

The pico base station 600 sets an interference reducing CCE according to the interference reducing CCE information notified by the macro base station 700 at S55.

As described above, the wireless communication system 3 of the fifth embodiment includes the pico base station 600 and the macro base station 700. The pico base station 600 includes the control unit 600 a. The control unit 600 a notifies the macro base station 700 of the information on the number of the interfered UEs, and sets the interference reducing CCE according to the interference reducing CCE information notified by the macro base station 700. The macro base station 700 includes the control unit 700 a. The control unit 700 a adjusts the number of the interference reducing CCEs based on the information on the number of the interfered UEs notified by the pico base station 600, and notifies the pico base station 600 of the interference reducing CCE information based on the number of the interference reducing CCEs. Therefore, even when the number of the interfered pico UEs in the picocell changes according to time or place, the wireless communication system 3 can ensure adequate resources for the PDCCHs of the interfered pico UEs in the picocell. Furthermore, the amount of the PDSCH REs to be muted in the macrocell can be reduced to a requisite minimum.

In the wireless communication system 3, the control unit 700 a of the macro base station 700 increases the number of the interference reducing CCEs when the number of the interfered UEs is equal to or greater than a predetermined value, and decreases the number of the interference reducing CCEs when the number of the interfered UEs is smaller than the predetermined value. Therefore, the number of the interference reducing CCEs is appropriately adjusted according to the communication status of the picocell. Consequently, it becomes possible to control the interference reducing CCEs in just proportion for each of the pico base stations.

In the above embodiments, the wireless communication system disclosed in the present application reduces interference between the macrocell and the picocell. However, the wireless communication systems 1, 2, and 3 are not limited to the above, and may be applied to a technology for reducing interference between a macrocell and a femtocell or interference between a picocell and a femtocell.

According to one embodiment of the wireless communication system disclosed in the present application, it is possible to reduce interference with a control channel while maintaining the reception characteristics of a data channel.

All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventors to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A wireless communication system that controls a transmission timing in each of cells so that a control channel of a first cell and a data channel of a second cell temporally overlap each other, wherein a base station of the first cell includes: a first control unit that notifies a base station of the second cell of information used to specify a resource of the control channel of the first cell, the resource corresponding to a predetermined resource unit; and a first communicating unit that transmits a control signal to a mobile station of the first cell by using a first resource of the control channel of the first cell, the first resource corresponding to at least a part of the predetermined resource unit and serving as a decoding object of the mobile station of the first cell, and the base station of the second cell includes: a second communicating unit that transmits a null symbol by using a second resource of the data channel of the second cell, the second resource corresponding to the predetermined resource unit, and the mobile station of the first cell includes: a third communicating unit that receives the control signal transmitted by the base station of the first cell via the first resource.
 2. The wireless communication system according to claim 1, wherein the predetermined resource unit is one or more control channel elements, and the first resource is a search space specific to the mobile station of the first cell.
 3. The wireless communication system according to claim 1, further comprising a plurality of the first cells, wherein the base station of the second cell further includes a second control unit that calculates, for each of the first cells, a resource to which a predetermined resource unit of each of the first cells is mapped, based on a wireless parameter of each of the first cells, and the second communicating unit of the base station of the second cell transmits, by a same subframe, a null symbol in all of the resources calculated by the second control unit.
 4. The wireless communication system according to claim 1, further comprising a plurality of the base stations of the first cells, wherein the first control unit of each of the base stations of the first cells notifies the base station of the second cell of information needed to specify a resource to which the predetermined resource unit is mapped and of information on the number of interfered mobile stations, the first communicating unit of each of the base stations of the first cells performs transmission by a control channel for a mobile station by using a predetermined resource unit in an overlapping region when a resource that is specific to the mobile station connected to a corresponding one of the first cells and that serves as a decoding object of the mobile station overlaps the predetermined resource unit in an interference reducing subframe for each of the first cells indicated by interference reducing subframe information notified by the base station of the second cell, the base station of the second cell further includes a second control unit that sets an interference reducing subframe for each of the first cells based on the information on the number of the interfered mobile stations notified by the base stations of the first cells, and that notifies the base stations of the first cells of the interference reducing subframe information, and the second communicating unit of the base station of the second cell transmits a null symbol in a resource to which a predetermined resource unit of each of the first cells is mapped in the interference reducing subframe for each of the first cells.
 5. The wireless communication system according to claim 4, wherein the first control unit of each of the base stations of the first cells notifies the base station of the second cell of information on the number of mobile stations that are in a wireless channel state of a predetermined quality or lower, as the information on the number of the interfered mobile stations.
 6. The wireless communication system according to claim 4, wherein the second control unit of the base station of the second cell sets a ratio between the first cells with regard to the number of the interference reducing subframes, based on a ratio between the first cells with regard to a value indicated by the information on the number of the interfered mobile stations.
 7. The wireless communication system according to claim 4, wherein the second control unit of the base station of the second cell sets the interference reducing subframes of the respective first cells so that the interference reducing subframes do not overlap each other.
 8. The wireless communication system according to claim 4, wherein the first control unit of each of the base stations of the first cells notifies the base station of the second cell of a wireless parameter of each of the first cells as information needed to specify a resource to which the predetermined resource unit is mapped, and the second control unit of the base station of the second cell specifies a resource to which a predetermined resource unit of each of the first cells is mapped, based on information on the wireless parameter notified by each of the base stations of the first cells.
 9. The wireless communication system according to claim 4, wherein the first control unit of each of the base stations of the first cells provisionally selects an aggregation level to be applied to a control channel for a mobile station connected to each of the first cells, based on a wireless channel state of the mobile station, and when a target resource that is specific to the mobile station corresponding to the provisional aggregation level of the mobile station and that serves as a decoding object does not overlap the predetermined resource unit, the first communicating unit of each of the base stations of the first cells selects and transmits an aggregation level, for which the target resource overlaps the predetermined resource unit and which is equal to or greater than the provisional value, or selects an aggregation level, for which the target resource overlaps the predetermined resource unit and which is smaller than the provisional value, and transmits the aggregation level by adjusting transmission power to be equal to or greater than a predetermined value.
 10. The wireless communication system according to claim 1, wherein the first control unit of the base station of the first cell notifies the base station of the second cell of the number of interfered mobile stations, and sets a predetermined resource unit according to information on an interference reducing resource unit notified by the base station of the second cell, and the second control unit of the base station of the second cell adjusts the number of predetermined resource units based on the number of the interfered mobile stations notified by the base station of the first cell, and notifies the base station of the first cell of information on an interference reducing resource unit based on the number of the predetermined resource units.
 11. The wireless communication system according to claim 10, wherein the second control unit of the base station of the second cell increases the number of the predetermined resource units when the number of the interfered mobile stations is equal to or greater than a predetermined value, and decreases the number of the predetermined resource units when the number of the interfered mobile stations is smaller than the predetermined value.
 12. The wireless communication system according to claim 4, wherein upon receiving a request for transmission of the null symbol from any of the base stations of the first cells, the second communicating unit of the base station of the second cell transmits a null symbol to a mobile station of the corresponding base station of the first cell.
 13. A base station of a first cell in a wireless communication system that controls a transmission timing in each cell so that a control channel of the first cell and a data channel of a second cell temporally overlap each other, the base station comprising: a control unit that notifies a base station of the second cell of information used to specify a resource of the control channel of the first cell, the resource corresponding to a predetermined resource unit; and a communicating unit that transmits a control signal to a mobile station of the first cell by using a first resource of the control channel of the first cell, the first resource corresponding to at least a part of the predetermined resource unit and serving as a decoding object of the mobile station of the first cell.
 14. A base station of a second cell in a wireless communication system that controls a transmission timing in each cell so that a control channel of a first cell and a data channel of the second cell temporally overlap each other, the base station comprising: a control unit that receives information used to specify a resource of the control channel of the first cell from the base station of the first cell, the resource corresponding to a predetermined resource unit; and a communicating unit that transmits a null symbol by using a second resource of the data channel of the second cell based on the received information, the second resource corresponding to the predetermined resource unit.
 15. A mobile station that performs a communication in a wireless communication system that controls a transmission timing in each cell so that a control channel of a first cell and a data channel of a second cell temporally overlap each other, the mobile station comprising: a communicating unit that receives a control signal transmitted by a base station of the first cell by using a first resource of the control channel of the first cell, the first resource corresponding to at least a part of a predetermined resource unit and serving as a decoding object of the mobile station of the first cell.
 16. A wireless communication method implemented by a wireless communication system for controlling a transmission timing in each cell so that a control channel of a first cell and a data channel of a second cell temporally overlap each other, the wireless communication method comprising: notifying, by a base station of the first cell, a base station of the second cell of information used to specify a resource of the control channel of the first cell, the resource corresponding to a predetermined resource unit; transmitting, by the base station of the first cell, a control signal to a mobile station of the first cell by using a first resource of the control channel of the first cell, the first resource corresponding to at least a part of the predetermined resource unit and serving as a decoding object of the mobile station of the first cell; transmitting, by the base station of the second cell, a null symbol by using a second resource of the data channel of the second cell, the second resource corresponding to the predetermined resource unit; receiving, by the mobile station of the first cell, the control signal transmitted by the base station of the first cell by using the first resource. 